1. Field of the Invention
The present invention relates to a process for producing a semiconductor device, more particularly to an improved selective oxidation process.
2. Description of the Prior Art
When a semiconductor device having a relatively thick oxide layer (e.g. a field oxide layer isolating a semiconductor element from other semiconductor elements) is produced, a selective oxidation process is usually utilized. Generally, the selective oxidation process comprises the steps of forming a silicon oxide layer on a semiconductor substrate, forming a silicon nitride layer on the oxide layer, selectively etching the nitride and oxide layers to expose a portion of the semiconductor substrate, and oxidizing the exposed portion of the semiconductor substrate. The above-mentioned selective oxidation process is explained in detail with reference to FIGS. 1A, 1B and 1C.
A silicon dioxide (SiO.sub.2) layer 2 (FIG. 1A) having a thickness of approximately 50 nm (i.e. 500 angstroms) is formed on a silicon (Si) substrate (i.e. a single crystalline silicon wafer) 1 by a conventional thermal oxidation method or chemical vapor deposition (CVD) method. Then, a silicon nitride (Si.sub.3 N.sub.4) layer 3 having a thickness of approximately 100 nm is deposited on the silicon oxide layer 2 by a conventional CVD method, as illustrated in FIG. 1A.
Next, the silicon nitride layer 3 and silicon dioxide layer 2 are selectively etched by a photoetching method, as illustrated in FIG. 1B. Namely, a photoresist layer (not illustrated) applied on the silicon nitride layer 3, is exposed and is developed to form a patterned resist mask. The unmasked portion of the silicon nitride layer is etched by a hot phosphoric acid solution and then the exposed portion of the silicon dioxide layer 2 is etched by a hydrofluoric acid solution to expose the underlying portion of the silicon substrate 1.
The silicon substrate 1 with the patterned silicon dioxide layer 2 and silicon nitride layer 3 then is subjected to a conventional thermal oxidation treatment to convert the exposed portion of the silicon substrate 1 into a relatively thick silicon dioxide layer 4, i.e. a field oxide layer or an isolation oxide layer, as illustrated in FIG. 1C. Simultaneously, an upper portion of the silicon nitride layer 3 is changed into a silicon dioxide layer 5 having a thickness of approximately 50 nm during the thermal oxidation treatment.
However, the above-mentioned selective oxidation process has the following disadvantages (1) and (2).
(1) If the silicon nitride layer 3 is directly deposited on the silicon substrate 1 without the silicon dioxide layer 2, when the substrate 1 is heated for the thermal oxidation treatment, internal stress (i.e. thermal stress) occurs at the boundary of the silicon nitride layer 3 and the silicon substrate 1, whereby cracks in the silicon nitride layer 3 and/or crystal faults in the silicon substrate are generated. As a result, the electrical properties of a semiconductor element to be formed are lowered. Therefore, in order to prevent the internal stress from occurring, it is necessary to provide the silicon dioxide layer 2 between the silicon substrate 1 and the silicon nitride layer 3. However, the silicon dioxide layer 2 promotes the formation of a so-called bird's beak, as illustrated in FIG. 1C. The bird's beak is formed by oxidizing a portion of the silicon substrate 1 which lies under the end portion of the silicon dioxide layer 2 so as to convert silicon into silicon dioxide during the thermal oxidation treatment. The bird's beak can reduce the dimensional precision of a semiconductor element to be formed in the region of the substrate 1 masked by the layers 2, 3 and 5 and bounded by the field oxide 4, lower the electrical properties of that semiconductor element.
(2) In order to remove the successive silicon dioxide layer 2, the silicon nitride layer 3, and the silicon dioxide layer 5 to expose the underlying portion of the silicon substrate 1 wherein a semiconductor element (e.g. a bipolar transistor, a metal oxide semiconductor field effect transistor (MOS FET) and the like) or a passive element (e.g. a diffused resistor) will be formed in the succeeding steps of the producing process for a semiconductor device, it is necessary to perform three successive steps. Namely, first the silicon dioxide layer 5 is etched by a hydrofluoric acid solution, secondly the silicon nitride layer 3 is etched by a hot phosphoric acid solution and lastly the silicon dioxide layer 2 is etched by a hydrofluoric acid solution. The above described three etching steps are not preferable from the viewpoints of production cost and production time.
It has been tried to employ a silicon oxynitride (SiON) layer, which is formed by a conventional chemical vapor deposition (CVD) method at a deposition temperature in the range of from 850.degree. to 1050.degree. C., as a barrier to oxidation. The silicon oxynitride layer is preferable to the silicon nitride (Si.sub.3 N.sub.4) such as layer 3 in FIG. 1C. However, etchrate of the silicon dioxide is larger than that of silicon oxynitride, so that a field oxide layer as provided by the silicon dioxide layer 4 in FIG. 1C is remarkably etched during removal of the silicon oxynitride layer by etching.